1. Technical Field of the Invention
The present invention relates to a back off control technology for an objective lens of an optical pick up when defocusing occurs in an optical disc apparatus.
2. Description of the Related Art
With the recent advances in high-density recording technology for optical discs, an increasing number of optical disc apparatuses use the optical system with a lens numerical aperture larger than that of the CD/DVD optical system in the conventional technology. When the lens numerical aperture is increased, a working distance (hereinafter referred to as a WD) between a surface of an optical disc (hereinafter referred to as an optical disc surface) and an objective lens is shorter, so that the both sides are approaching each other and are likely to come into contact with each other during recording or reproducing. The optical disc surface and the objective lens surface are damaged when contacting each other. For example, in the case of a 3-wavelength optical system that shares CD/DVD optical systems and a blue laser optical system, the WD in the recording or reproduction of a CD is the shortest, so that the optical disc surface and the objective lens surface are likely to contact (abut) each other. Thus, there is required a countermeasure technology that can avoid the contact when defocusing occurs due to an increasing surface deflection of the optical disc or other factors.
There are technologies relating to the above-described countermeasure technology, which are described in Japanese Patent No. 3380832 and Japanese Patent Publication Laid-Open No. 2005-166088. In JP-B-3380832 there is described a technology that detects a total signal from the reflected light of the optical disc, and makes the objective lens back off in a direction to separate from the optical disc by an actuator when the total signal is smaller than a judgment value. In JP-A-2005-166088 there is described a technology that enables the secure tracking control and focus control by suppressing the influence of the surface deflection of the optical disc. In order to achieve this, the surface deflection of the optical disc is detected by a focus error signal at a lower recording speed, and the amount of applied bias is determined based on the detected surface deflection, and in accordance with a bias signal based on this, a constant distance between the convergence lens and the optical disc is maintained to assure an accurate focus servo also in the case of high-speed recording.